(a) Field of the Invention
The present invention relates to a method of synthesizing a phosphorus compound which is used as a raw material in a pull-up production of monocrystal compound semiconductor, etc., and more particularly to a method of synthesizing Group III element-phosphorus monocrystal compounds in which a Group III element and phosphorus are directly synthesized together.
(b) Description of the Prior Art
GaP monocrystal is a semiconductor having a relatively large energy gap among others of the Group III-V compound semiconductor, and allows an easy control of the n and p conductive type control is relatively simple, and is therefore used as a raw material of light emitting diodes. However, the production of GaP offers various problems in synthesis, and therefore the production of GaP monocrystal has been extremely difficult. That is, phsosphide has a high melting temperature, as is well known, and more over it exhibits a high decomposition pressure at that melting temperature. Accordingly, in the conventional method, phosphorus is vaporized so that the compound thereof decomposes causing it difficult to produce GaP monocrystal.
Recently, a method generally known as "a high pressure pull-up method under a liquid encapsulate", has become applicable in the industrial production of monocrystal of phosphide such as, for example, GaP, etc.. In this method, the top of GaP polycrystal which has been synthesized beforehand and received in a crucible is covered with B.sub.2 O.sub.3 having a low vapor pressure, and is then pressurized with the use of inert gas within the high pressure container. Under this condition the crucible is heated to melt GaP. At this stage B.sub.2 O.sub.3 covers the top surface of GaP to prevent phosphorus from vaporizing. In this condition GaP monocrystal is pulled up through B.sub.2 O.sub.3 (an encapsulate) to allow the growth of GaP monocrystals.
However, in this method, the GaP polycrystal should be preliminarily synthesized by other means. Accordingly, in order to manufacture a high quality GaP monocrystal at a low cost, it is necessary to prepare a high quality GaP polycrystal at a low cost.
By the way, there have been proposed low pressure synthesis method for synthesizing GaP polycrystal. For example, in one of the methods, the gas of H.sub.2 +PH.sub.3 is allowed to flow over Ga in a boat disposed within a heating furnace while Ga is heated up to a temperature of about 1000.degree. C. GaP then is gradually synthesized from one end of the boat by inducing a temperature difference of 100.degree. C. between both ends of the boat. In another of the methods, GaPO.sub.4 is fired and reduced in a hydrogen atmosphere at a temperature of 850.degree. C. However, GaP polycrystals obtained by these methods are powder-like, so that the GaP polycrystals thus obtained should be further processed into a high-density before they are pull up to form a monocrystal thereof, resulting in that the steps are complicated and the price of the product increases.
Meanwhile, there has also been proposed a high pressure synthesis method of high dense GaP polycrystal suitable for pulling up monocrystal, in which Ga is heated under a high vapor pressure of phosphorus to synthesize GaP polycrystal. In this method, red phosphorus is disposed in an open end of a high-pressure resistant sealed pipe laid in an electrical furnace, while a container disposed therein with Ga is arranged at the other end of the sealed pipe, then red phosphorus is heated up to a temperature of about 600.degree. C. to saturate the inside of the sealed pipe with the phosphorus vapor. The vapor is then reacted with Ga which has been locally heated up to a temperature near the melting point of GaP (up to 1467.degree. C.), thereby gradually synthesizing GaP polycrystal. However, although this high-pressure method may produce highly-dense GaP polycrystal, it is accompanied with a disadvantage in that a stoichometric composition cannot be obtained since the control of gas-pressure of phosphorus vapor is very difficult.
Further, both low and high pressure synthesis methods mentioned above require a long time (several days) for the synthesis of GaP polycrystal, and therefore, not only incurs low productivity and an increase in cost but also presents the problem of contamination in a reaction system since the reaction system must be maintained at a high temperature for a long time.
It is noted here that the above-mentioned problems are not limited to the production of GaP but also with the production of other Group III-V compound semiconductor, such as, for example, InP, GaAsP, etc. or a compound of Group III element and phosphorus.